As a method wherein a stabilizer is added before polymerization or during polymerization of olefins, Patent Document 1 proposes a method wherein olefin monomers are polymerized in the presence of a phenolic antioxidant. A method wherein a phenolic antioxidant is added before the polymerization or during the polymerization has the advantage of being able to omit a step of blending of the stabilizer by melt-kneading such as extruding after the polymerization, but it has been suggested that phenolic antioxidants having an ester bond such as tetrakis(3-(3,5-di-tert-butyl-4-hydroxyphenyl)propionyloxymethyl)methane have problems in that the catalytic activity of a polymerization catalyst may decreased; the interaction between a stabilizer and a metal in a polymerization catalyst may cause coloring of olefins; and the like, so that selection and control of polymerization conditions are laborious, which is problematic.
The present inventors reported in Patent Document 2, Patent Document 3 and Patent Document 4 that masking of a specific phenolic antioxidant by an organoaluminum compound can solve the above-described problems. However, although a phenolic antioxidant masked by an organoaluminum compound suppresses adverse effects on a polymerization catalyst, the effect of the action of the phenolic antioxidant as a stabilizer cannot be obtained in cases where the masked phenolic antioxidant is used as it is, so that a step of recycling of the masked phenolic antioxidant to a phenolic body is required after polymerization of olefin monomers.
In Patent Documents 2 to 4, methods wherein a phenolic antioxidant is recycled by an alcohol, water or water vapor employed in a step of removal of the polymerization catalyst residue after completion of a polymerization step are proposed. Actually, by sufficiently carrying out the polymerization catalyst deactivation treatment after completion of batch polymerization with an alcohol, water or water vapor, a masked phenolic antioxidant can be recycled to a phenolic body and a sufficient stabilization effect can therefore be achieved.
Further, a method for producing a fiber by the melt-blown method using a polyester polymer is known. The melt-blown method is a method wherein a molten resin is extruded to form a small resin flow, which is then brought into contact with a high-speed heated gas to be made into a discontinuous fiber having a small fiber diameter, followed by accumulating this fiber on a porous support, to obtain a fiber. By using the melt-blown method, productivity of polyester resin fibers has been improved. Use of the melt-blown method is being studied also for other various materials, and its use has been attempted also for polyolefins.
Polyolefins have a problem in that they have poor stability to heat and light and hence are easily oxidized/deteriorated when the polyolefins are processed at a high temperature and when they are exposed to heat and/or light during their use, resulting in insufficient service lives as plastic products. Thus, in general, in order to prevent oxidation/deterioration, a stabilizer(s) such as a phenolic antioxidant, phosphorus-containing antioxidant, sulfur-containing antioxidant, hydroxylamine compound, hindered amine compound, ultraviolet absorber and/or acid scavenger is/are added. Selection, blending and the like of various stabilizers depending on the intended use are being studied to realize their practical uses.
In cases where a stabilizer is blended in a polyolefin, a method wherein the polyolefin and the stabilizer are mixed together and granulation is then carried out using an extrusion apparatus or the like is generally employed, and, for example, Patent Document 5 and Patent Document 6 propose polyolefin resin compositions prepared by inclusion of a 1,3,5-tris-(4-t-butyl-3-hydroxy-2,6-dimethylbenzyl)isocyanurate phenolic antioxidant and a phosphorus-containing antioxidant in a polyolefin resin.
However, in cases where a fiber is to be obtained by the melt-blown method, there is a problem in that, since the polyolefin employed has extremely high fluidity (in some cases, a melt flow rate of 200 to 600 g/10 min.), granulation is impossible with a usual extrusion apparatus.
As a method for stabilizing a polyolefin wherein a granulation step using an extrusion apparatus is not employed, methods wherein a stabilizer is added before polymerization or after polymerization of olefin monomers have been studied. For example, in Patent Document 7, a production method wherein α-olefins are polymerized in the presence of a phosphorus-containing antioxidant is shown. It is shown that, when compared to polyolefins produced by mixing an antioxidant with a polymer obtained by polymerization of α-olefins without using a phosphorus-containing antioxidant, polyolefins produced by polymerizing α-olefins in the presence of a phosphorus-containing antioxidant have more excellent stabilization effects.
Although a method wherein a stabilizer is added before polymerization or after polymerization of olefin monomers has the advantage of being able to omit a step of blending of the stabilizer by melt-kneading such as extruding after the polymerization, the method has problems in that the catalytic activity of a polymerization catalyst may be decreased; the interaction between a stabilizer and a catalyst metal may cause coloring of the polyolefin resin; and the like, so that selection and control of polymerization conditions are laborious, which is problematic.
Examples of particular influences of the stabilizer on the polymerization catalyst include, as suggested in Patent Document 1, lowering of the catalytic activities of polymerization catalysts by phenolic antioxidants having an ester bond, such as tetrakis[3-(3,5-di-tert-butyl-4-hydroxyphenyl)propionyloxymethyl]methane, which are stabilizers widely used for polyolefins. Thus, it has been shown that such phenolic antioxidants are not suitable for being added before the polymerization.
As methods for avoiding such influences on polymerization catalysts, Patent Document 3 and Patent Document 4 describe methods wherein a phenolic antioxidant masked by an organoaluminum compound is added before polymerization or during polymerization of monomers having an ethylenically unsaturated bond, to produce a stabilized polymer.